Most places have laws about what you can do with used motor oil, such as mandating that it be recyeled* by a licensed facility. In fact, while writing this it was initially difficult to find any information on used motor oil other than pleas to recyele it. Used motor oil is a known carcinogen, so wear gloves and don't eat it.
A lot of the pages on the net that talk about used motor oil take the amount of oil bought and subtract the amount recycled, concluding that this amount is dumped down storm sewers or into lakes and streams. But the thing is, most people don't live near a lake or a stream, and very few people would actually dump used motor oil down the storm sewer, much less their own drain. At least, not twice.
So what do people do with used motor oil if they don't recyele it?
- Paint the bottom of wooden posts, or anything else that will be in ground contact. If you're really serious, heat the oil to above the boiling point for water and immerse your object in it. The water in the object will boil away, to be replaced by oil and contaminants. Oiled wooden fence posts that are mostly or entirely encased in concrete will not harm the environment. Don't use such posts in standing water or marsh, because the oil will leech into the water.
[Update: I found this research report from Donald J. Miller of Oregon State University, Service Life of Treated and Untreated Fence Posts: 1985 Post-Farm Report. "Post-Farm" refers to a farm full of posts, not a report after the farm.
He recommends used motor oil or crankcase oil only as a carrier for creosote, presumably because the oil only increases the useful life of posts by a short amount of time compared to its toxicity (and tendency to soften untreated wood). Using oil to carry creosote proved among the most effective treatments. In a residential or non-farm application, applied to already treated wood, plain appears to help a lot. /Update 20070728]
- Soak hard-to-burn things like damp wood before lighting (don't breathe the smoke)
- Paint concrete (again, not for aquatic use).
- Paint concrete forms so they can be removed more easily taken off and reused
- Paint a brick foundation, especially the chalky mortar as a way to postpone tuck pointing. This iwill make your eventual tuck pointing job more difficult, since the new mortar will not stick as well with the oil present.
- Before paving asphalt (or asphalt over concrete), apply a coat of motor oil and allow to soak in overnight (unless rain is expected).
- Mix with staining colorants, or with old oil-based wood stain or paint. A lot of times there are things that need to be painted that will never show -- you're just painting them to protect them (why else use an oil-based paint?). So combine your leftover stain, paint, varnish, and used motor oil in a paint can and mix well. It probably won't stick to metal, but will last a long time on wood, concrete, or stone. For an effective finish, apply several coats until it is no longer absorbed.
- Use as chain saw chain oil (a very good use, but watch the splatter)
- Lubricate a bicycle chain (this is not optimal, because the thick oil tends to collect dirt)
- Clean rusty metal (paired with an abrasive such as steel wool)
- Coat outdoor tools such as axes, shovels, and clippers to keep them from rusting
- Clean road tar off vehicles (take care not to mar finish)
- Clean old engines such as on lawnmowers
- Lubricate hinges, latches, and other rough exterior hardware
- Coat nails to make them drive easier and stay rust-free
- Mix in 1 part oil with 20 parts diesel fuel and drive around †
- Oil leather (makes a dull, unattractive finish, but functions well) (do not used next to skin)
- There is a patented process for cooking used rubber tires with used motor oil, producing a variety of hydrocarbons and chemicals.
- Another (patented) process purifies used oil with acetone and ketone
- Use a centrifuge to separate impurities †
- Burn in an oil heater designed for used oil†
- Burn it in an oil lamp†
- Spread it on the ground to inhibit dust or weed growth, as this makes a nasty runoff when it eventually rains.
- Ingest it or let it contact your eyes or other such areas.
- Dump it down the drain, as this will clog your drain and mess up the sewer system
- Put it in a landfill, as this is dangerous and harmful in about six ways.
* I have left the spelling as "recyeled" to allow searchers to find this post by excluding the correctly spelled word.
† ← I've never tried these, but they should be interesting